The avian song system has proven to be a rich source of information on such general issues as neural sexual dimorphism, effects of steroids on brain development and function, and adult neural plasticity. The purposed research will integrate sophisticated microanatomical techniques with hormonal manipulation and behavioral observation in canaries and zebra finches in order to further study these issues. The first set of experiments will measure dendritic and synaptic change accross development in two song system regions. This experiment should indicate how additive and regressive anatomical changes interact to produce major dimorphisms between the sexes. This experiment will also assess whether abrupt changes in anatomy of song related regions occur in parallel to major changes in vocal behavior. Two other expriments will determine whether synaptic degeneration precedes regressive events in zebra finches. The site and amount of degenation will be measured so as to determine whether this potential signal is foccussed or difuse. This experiment should delinate very percisely the location and nature of synaptic events associated with differentiation. A fourth experiment will trace the projection between two nuclei that become very dimorphic, in very young zebra finches. It appears that steroids cannot act directly on one of these nuclei-differentiation here must involve indirect mediation. Such a signal could be conveyed by the pattern of afferentation, especially if the axonal projection to this region is larger in males than in females, at the time differentiation is occuring. Testosterone will be given to adult female canaries so as to induce singing. The consequences of this treatment will be assessed in the motor nucleus that controls the vocal organ. This experiment will test whether areas within the nucleus that differ in function also differ in their response to the steroid, and whether a consequence of the treatment is to create left-right asymmetries anatomy. Finally, hormone- induced song will be used to determine the effects of damage in a song- related brain region on song acquisition and performance. These experiments address issues closely related to human health. Endocrine events are known to have powerful effects on human brain development. Humans have sensitive periods of r some sorts of learning or sensory development for which the initiating and terminating events are poorly understood. Humans may have predispositions for vocal learning which are similar to the "experience-expectant" state of the juvenile songbird. Human behavior frequently involves integration of many endocrine, developmental and experiential influences. The song system is an ideal preparation in which such disparate influences on a complex learned behavior may be dissociated and understood.